1. Field of the Invention
The present invention relates to a wavelength conversion method that converts the wavelength of a coherent light beam or beams using a nonlinear optical wavelength conversion element, the main object thereof being to improve the conversion efficiency for the case where some of the power involved is absorbed in the conversion element.
2. Description of the Prior Art
It is known that lasers and nonlinear optics can be used in combination to produce coherent radiation at other wavelengths than the fundamental wavelengths produced by the lasers. One example is harmonic generation, where the new wavelengths are an integer division of the fundamental wavelengths. Another example is sum-frequency generation, where the new wavelength is equivalent to the sum of the frequencies (speed of light/wavelength) of two lasers. A further example is difference-frequency generation, where the new wavelength is equivalent to the difference of the frequencies of two lasers. It is also possible to have cascaded systems where the nonlinear-generated beam or beams drives another nonlinear-generation scheme. Such nonlinear optics can include nonlinear crystals in which the conversion of wavelengths occurs. For example, crystals with symmetries that lack a center of inversion can effectively generate a wavelength that is half the laser wavelength, a process known as second-harmonic generation, since they can produce output that is proportional to the square of the electric field of the laser beam inside the crystal. This so-called second-order nonlinearity can also be used for sum-or difference-frequency generation. We refer to the laser beam or beams (or beams that result from another nonlinear-generation process) as the input beam or beams and the resultant nonlinear-generated beam as the output beam.
In order to obtain efficient harmonic, sum- or difference-frequency generation in a nonlinear crystal, it is necessary to arrange the electric fields of the input and output beams to be in phase with each other as they propagate in the crystal. If they are, the power of the output beam can grow along the length of the crystal and lead to efficient conversion of energy from that of the input beam or beams, provided that the intensity (power/unit area) of the beam or beams is sufficiently high. If the beams do not stay perfectly in phase, the power in the output beam can convert back into power in the input beam or beams, and the efficiency (output beam power/input beam or beams power) is reduced, in the worst case to no conversion at all. The process of keeping the beams in phase is commonly referred to as phase-matching.
In crystals, phase-matching can be accomplished, for example, for beams all propagating along the same direction, if the refractive indices for all wavelengths involved are the same. However, the refractive indices are in general a function of wavelength. Throughout the near-infrared, visible and ultraviolet (UV) wavelength regions, most crystals (for a given polarization and propagation direction in the crystal) exhibit normal dispersion, an increase in refractive index with decreasing wavelength. Thus, phase-matching can only be achieved through the use of birefringent crystals in which the refractive indices are also a function of the polarization of the beam and the direction of propagation in the crystal. By the appropriate choice of polarization and beam propagation direction in the crystal for the input beam or beams and output beam, it is possible in many nonlinear crystals to obtain phase-matching. In the case in which the all the beams propagate along one of the principal axes of the crystal, the process on non-critical phase-matching is said to occur. In all other cases, the process is referred to as critical phase-matching.
It is well known that the refractive indices in crystals in general change with changes in crystal temperature. This can be used to advantage in nonlinear optical systems, since the temperature can often be adjusted to achieve exact phase-matching. However, the change of refraction with temperature can also present a practical problem. If the nonlinear crystal is adjusted for exact phase-matching at one temperature, the nonlinear conversion efficiency will drop if the temperature varies, by an amount that depends on the particular characteristics of the crystal.
Nonlinear crystals absorb some of the power from the input beam or beams and from the output beam. The absorption can be due to several processes, including absorption from electronic or vibrational transitions inherent in the crystal, absorption from impurities or defects in the crystal, nonlinear effects such as two-photon absorption and more complex phenomena such as absorption from transient crystal defects, or color centers, created by the input or output beams. Absorption has one direct effect, the reduction in the power of the output beam. There is a second, indirect effect that often is more significant. The absorbed power leads to heating in the nonlinear crystal, and the subsequent change in the crystal temperature can result in a loss of perfect phase-matching, reducing the power in the output beam. Even when the absorption is small enough to cause only a minor direct loss of output power, the indirect effect of heating and subsequent loss of exact phase-matching can cause a drastic reduction in output power. If the temperature rise was uniform through the volume of the nonlinear crystal, the crystal orientation could be adjusted to compensate, or the crystal temperature could be adjusted through means of external heaters or chillers. In practice, the dynamical nature of the heating can make compensation for heating difficult to implement. The problem is particularly challenging when the output power is responsible for creating the heating. Also, the heating is generally not uniform throughout the volume of the crystal, due to the nature of the absorption and the spatial variation of the power in the laser and output beams. Thus, correction for the heating effect is generally incapable of eliminating all the reduction in output power.
One other effect that can reduce nonlinear power is some crystals is photo-refraction. In this effect, the laser or nonlinear-generated beams create defects in the nonlinear crystal that have electrical charge. The resultant electric field produced in the material changes the refractive index through the electro-optic effect, and that can lead to destruction of perfect phase-matching. As with heating, the effect is difficult to correct because of non-uniformities in the input or output beams and the resultant non-uniformities in the photo-refraction effect in the crystal.
The recognition of the effect of heating in the nonlinear crystal has led to several techniques to compensate for it.
U.S. Pat. No. 4,019,159 issued Apr. 19, 1977 to Hon et al. describes a method using an electric field to control the refractive indices of a nonlinear crystal and compensate for the effects of heating.
U.S. Pat. No. 4,181,899 issued Jan. 1, 1980 to Liu shows a device where the temperature of the nonlinear crystal is monitored electronically, and through control electronics and a voltage-controlled tuning element, the wavelength of the laser driving nonlinear crystal is adjusted to maintain phase-matching as the crystal temperature changes due to heating.
U.S. Pat. No. 5,898,718 issued to Mohatt et al. teaches a crystal heater design that establishes a gradient in temperature along the length of the nonlinear crystal, in part to compensate for non-uniform heating from the second-harmonic output beam.
U.S. Pat. No. 6,744,547 B2 issued to Ikeda et al. describes a temperature control method for a nonlinear crystal that adjusts the crystal temperature to correct for changes in the average power of the input beam.
A book chapter by Hon (D. Hon, “High average power, efficient second harmonic generation,” Chapter B2 in Laser Handbook, Volume 3, Ed. M. L. Stitch, North-Holland Pub. Co., Amsterdam, N.Y., 1979) describes a number of techniques to generate high second-harmonic powers in the presence of nonlinear crystal heating.
However, all the techniques attempt to correct for the heating effects after they occur in the crystal and do not generally address means to reduce the effects in the crystal itself. Since the techniques discussed 1) add more complexity to the overall nonlinear conversion system and 2) are in general not fully effective in eliminating the reduction in nonlinear conversion there is a need for other means to reduce heating and other deleterious effects in nonlinear crystals.